[dnsdist] Question on backend selection

[Tom]

Hi list

Using dnsdist 1.5.2 with four configured backend servers 
(backend01-backend04).

Is there a way to configure dnsdist to use backend03 and backend04 only, 
if backend01 and backend02 are not answering on the healthcheck? Or 
better: use backend03 or backend04 only if backend01 or backend02 are down.

Any hints?

Thank you.
Kind regards,
Tom